44.
Weight/Weightlessness
(Phil Physiker contrasts mg with and without a
support force.)
End Question: If you were in a freely-falling elevator and
you dropped a pencil, it would hover in front of you. Is
there a force of gravity acting on the pencil? Defend your
answer.

45. Gravity
(Paul shows how the equation for gravity is a guide to
thinking.)
End Question: Pretend you’re in a spaceship that passes a
giant star and you feel its gravitational pull. Then the
star collapses to become a black hole. How, if at all, will
your gravitational pull to the black hole compare?

46. Gravity Inside
Earth
(Application of the gravitational equation to Earth’s
interior is treated.)
End Question: In a cavity at Earth’s center, do you
experience the gravitational pull of the Sun?

47. Tunnel Through
Earth
(Paul considers a tunnel bored through Earth and your motion
if you were to fall into it.)
End Question: When you whiz past the center of the tunnel
through Earth, you have maximum speed. How does this speed
compare with the constant speed of the satellite in close
circular orbit? Think about that.

48. Ocean Tides
(Paul explains how the Moon, and also the Sun, cause ocean
tides.)
End Question: Suppose you go to the beach to dig clams at
the time of an extra-low tide. Is it reasonable to assume
that in about six hours the tide that follows will be extra
high?